Active Virtual Fence Using Mesh Networked RF Tags

ABSTRACT

A system capable of monitoring an area and detecting a disturbance with the area. The system has a plurality of ultra-wide band radio frequency tags, each of the tags including a digital signal processing module configured to monitor changes in radio frequency multipath properties of received packets transmitted by at least one other of the tags. Changes in the radio frequency multipath properties may be caused by a disturbance indicative of an object in the vicinity of the tags. A corresponding method is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/530,223, filed on Jan. 14, 2010, which is the U.S. National Stage ofInternational Application No. PCT/IL2008/00424, filed on Mar. 27, 2008,which claims the benefit of U.S. Provisional Patent Application No.60/908,696, filed Mar. 29, 2007, the contents of each are incorporatedby reference herein in their entirety.

FIELD AND BACKGROUND

The present invention relates to virtual fences and, in particular, itconcerns an active virtual fence using mesh networked RF Tags.

There are many security applications in which a sensitive area must beprotected from intruders. A conventional approach is a wall or a fence.However, advances in current technological allow the use of anelectronic fence or virtual fence which senses the presence of unwantedactivity in the protected area and usually generates a warning.

Current approaches for building a virtual fence include placing an arrayof control towers, each equipped with movement detection and securityradar, cameras or motion detectors. Such as the systems disclosed inU.S. Patent Application No. 2006/0033617 to Wakefield and U.S. Pat. No.4,124,848 to Clark, et al. A similar system was described at thebeginning of 2007 by the US Department of Homeland Security with regardto awarding aerospace giant Boeing a contract to provide high-techmethods to catch illegal immigrants along the U.S.-Mexico border.Boeing's “virtual fence” concept includes an estimated 1,800 towersdeployed along the border and equipped with cameras and motion sensors.The cost of this project is about 10 billion dollars.

Another approach offered by some companies is a Virtual Fence PassiveMulti-Sensor System, which includes a covert, passive device designed todetect human activity independent of a physical obstacle. This VirtualFence is a sensor array, combining acoustic, seismic and infrared (IR)sensors to detect, locate, and identify human activity in a protectedarea. The system enables target detection beyond line of sight, such asmovements of humans, vehicles and low flying helicopters hidden inground defilades.

In such a system, the sensors are installed underground, in sealed boxescontaining digital signal processors and wireless communicationsdevices, geophones, microphones, IR detectors and power sources. Theonly element above ground is a concealed camouflaged antenna and IRdetector. Each sensor contains a processing unit which performs targetidentification (ID) and direction finding (DF). The IDDF algorithmsenable monitoring and discriminating between diverse activities, such asmoving humans, digging and building work, as well as mechanical noisessuch as approaching vehicles. The system can be implemented to provideearly warning for border and perimeter control, tunnel excavation,protection of pipelines and electrical infrastructure, guarding vitalinstallation perimeters as well as monitoring prison fences.

Other previous art virtual fence techniques have been described in U.S.Pat. No. 6,067,044 Whelan, et al., and U.S. Pat. No. 6,342,847 toArchuleta, et al., and U.S. Patent Applications Nos. 2006/0197672 toTalamas et al. and 2006/0238340 to Harvey. These virtual fences use GPSto determine if a tagged object is outside an allowed boundary. The GPSis used to measure the location of the object and this location iscompared to a threshold defining the perimeter of the virtual fence.

Other patents suggest the use earth based transmitters for locating anobject. Examples include U.S. Patent Applications No. 2004/0108939 toGiunta in which the device uses both GPS as well as earth basedtransmitters to locate itself. U.S. Pat. No. 6,353,390 to Beri et al.uses cellular base stations for location. U.S. Patent Application No.2007/0257831 to Mathews et al. uses CDMA signals from earth basedtransmitters. U.S. Pat. No. 6,720,881 to Halliday describes a wearabletransmitter, an alarm condition is detected when the transmitter isdetected by some receivers in the perimeter. All these solutions requirethat the tracked object/person will carry a special unit used forlocation. This requirement is not feasible in security applications inwhich the objective is to determine the presence of a foreign intruder.

Other suggested techniques for creating a virtual fence include usinglaser beam or infrared light, such as in U.S. Patent Application No.2007/0012901 to Mikuski et al. and U.S. Pat. No 6,985,212 to Jamieson etal. in which a fence using infrared light is made, such that an intruderis detected if the beam is broken. Such approaches are possible only forshort distance over flat terrain and are dependent on weatherconditions.

Two other proposed solutions are provided in U.S. Patent Application No.2005/0147340 to Tapanes that describes fibers that are buried in theground to detect when they are being stepped on, and U.S. PatentApplication No. 2004/0113780 to Pottratz which suggests tripwires todetect crossing.

There is therefore a need for a virtual fence using mesh networked RFTags.

BRIEF SUMMARY

The present invention is a virtual fence using mesh networked RF Tags.

According to the teachings of the present invention there is provided,[TO BE COPIED IN FROM CLAIMS WHEN FINALIZED]

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a schematic block diagram of a virtual fence constructed andoperational according to the teachings of the present invention;

FIG. 1A is a schematic block diagram of a virtual fence in its simplestform constructed and operational according to the teachings of thepresent invention;

FIG. 2 is a schematic block diagram of a plurality of mesh networked RFTags in a virtual fence according to the teachings of the presentinvention;

FIG. 3 is a schematic graphic representation of the multipath componentsreceived by one RF Tag of the plurality of Tags in FIG. 2 when nodisturbance is present within the area of the plurality of Tags;

FIG. 4 is a schematic block diagram of the plurality of mesh networkedRF Tags of FIG. 2, shown with an intruder;

FIG. 5 is a schematic graphic representation of the multipath componentsreceived by the RF Tag of FIG. 3 when the intruder of FIG. 4 is presentwithin the area of the plurality of Tags; and

FIG. 6 is a schematic block diagram of a method for determining thelocation of the intruder of FIG. 4 according to the teachings of thepresent invention.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

DESCRIPTION

The present invention is a virtual fence using mesh networked RF Tags.

The principles and operation of a virtual fence according to the presentinvention may be better understood with reference to the drawings andthe accompanying description.

By way of introduction, the present invention provides an Active VirtualFence Multi-Sensor System, employing wide-band RF tags which are capableof mesh networking.

The invention uses the same RF tags used for RFID applications asdescribed in patent applications “Method and system for distancedetermination of RF tags (PCT/IL2003/000358) and “Communication anddistance measurement in an addressed wide-band RFID system”(PCT/IL2005/000967), with additional features which adapt the RF tags tothe new applications, without much increase in cost.

The innovative system is based on two concepts.

The first concept is the use of area covering sensors which detect adisturbance indicative of the presence of objects (both moving and notmoving) in the area of the sensor using a multipath signature of thereceived signal. Unlike RADAR which sends a signal and looks for itsreflection, the present invention uses only the RF link between two ormore nodes such as, but not limited to, tags and readers. No additionalRF circuits are needed, which means very low cost devices. As usedherein, the term “disturbance” refers to a change in the multipathsignature (properties) of the signals received by at least one node inthe system.

The second concept is mesh networking of all the Tags in the systemtogether to minimize the need for a dense fixed infrastructure forcommunicating with the control center. The protocols for implementing anAd-hoc mesh networking of wireless sensors is known in the art anddescribed in various academic papers. The resulting tag can beimplemented on a small single chip, using a very small battery with alife span of several years and without any maintenance. Such a tag issmall enough to be concealed inside a small innocent looking stone inthe field. The tags can even be spread from the air and immediately formthe ad-hoc mesh network between them. In addition, each tag has theability to measure the round trip delay to its neighbor by exchangingpackets as described in patent application PCT/IL2003/000358 “Method andSystem for Distance Determination of RF Tags.”

All such measurements, including the tags identity information arepassed through the mesh network until they reach a fixed reader whichcollects the information. The readers further send their data to acentral processor which will build a two-dimensional mapping of the tagsas randomly placed to cover the area. Preferably, the (x,y) coordinatesof each tag and its nearest neighbors should be stored in each tag.

In some military applications, such tags may be spread by a special kindof missile or bomb that spreads them all over enemy area.

Referring now to the drawings, FIG. 1 illustrates a virtual fence system2 containing a mesh of tags 4 with one or more fixed readers 6. Thevirtual fence of FIG. 1 is created by placing a belt of tags along thelength of the area to be monitored. The tags 4 can be placed in anorderly fashion such as, by non-limiting example, on some grid asillustrated in section 10, randomly as in section 20 or in a straightline as illustrated in FIG. 1A.

If a straight line pattern is used, a minimum number of tags arenecessary. However, distributing the tags in a belt pattern willincrease the reliability of the fence since many more tags will have anoption to detect an intruder or other disturbance. The wider the belt,the longer the intruder can be tracked. Further, the use of a beltdistribution reduces the probability of false alarm. Furthermore, thedirection of movement will be more evident by observing the relativetiming of the detection. If the first row of tags 4 a is first to detecta disturbance and then the second row of tags 4 b detects a disturbance,it can be concluded that the object causing the disturbance is movingtoward the area of the virtual fence 2. The direction being traveled bythe object causing the disturbance can also be determined by monitoringwhich tags 4 detect the disturbance and the order in which the tags 4detected the disturbance. This information can not be provided by thevirtual fence of FIG. 1A.

The operation of the preferred embodiment is as follows. As illustratedin FIG. 2, each tag 4 in virtual fence 2 transmits packets to itsneighboring tags 4. The neighboring tags analyze the packet, asillustration by tag 40, and estimate the multipath properties from thereceived signal as seen in FIG. 3. The multipath properties are afunction of the environment around the transmitter and the receiver.Some of the multipath components may be reflections from objects. Thenon-limiting example of FIG. 3 illustrates the multipath componentsreceived by tag 40 from its neighboring tags 4 c, 4 d and 4 e. In astable environment the multipath properties should remain substantiallyunchanged over time.

In FIG. 4 an intruder 30 has entered the area of the virtual fence 2 anda new multipath component 32 is now being received by tag 40. It shouldbe noted that a new multipath component will also be received by tag 4 cand tag 4 f will no longer be receiving the multipath component form tag4 d.

In one embodiment, each tag includes a Digital Signal Processor foranalyzing the multipath properties. Alternatively, each tag 4 may usethe mesh network to transmit the estimated multipath properties to adifferent tag that contains a DSP or up to a reader or control processorwhich analyzes the data. In the latter cases, it is advantageous thatthe tag have the minimal capabilities to differentiate between thecurrent multipath and previously estimated multipath from same sender inorder to save transmission bandwidth. The function of the DSP is todetect new multipath components or changing multipath components. Asshown in FIG. 3, there is a multipath pattern, before the intruderentered the area. After the intruder 30 entered the area of the virtualfence 2, FIG. 5 shows the newly created multipath component 32 a beingreceived by tag 40. In other cases, such as the communication betweentags 4 d and 4 f, with no graph shown, the intruder will reduce amultipath component by blocking it. The DSP can estimate not only thepresence of a disturbance, but also to classify it. The data collectedin one tag is sufficient for estimating the size and the material of theobject by observing the height of the multipath component. The rate ofchange in the multipath component is an indication of the speed of theobject. For example it is possible to distinguish between rain, birdsand a person. The rain will cause an overall decrease in power in allmultipath, components. The bird is too small to cause any change. Aperson will form a distinct new multipath component. A Vehicle willregister as a much stronger multipath component than that of a person.The location and direction of movement of the object can be determinedby intersecting the data from at least two tags, or by one tag that hascollected multipath patterns from at least two transmitters. Upondetection of a disturbance within the mesh network, the CentralProcessing Unit (FIG. 1, 40) provides detailed information regarding thedisturbance such as, but non-limited to, time of detection, location ofdetection, current location, estimated size of the object causing thedisturbance, and direction and speed of travel through the mesh networkthat forms the virtual fence.

As illustrates in FIG. 6, if two tags T2 and T3 transmit their estimatedmultipath pattern to a transmitting tag T1, T1 can estimate the relativeposition of the object 34. Having the tags measure the relative distancebetween them and that a line of sight component exists between them, T3can measure d1+d2 and T2 can measure d1+d3. This is based on receivingboth the direct path from T1 as well as the reflected wave from theobject 34.

Each of the measurements d1+d2 by T3 and d1+d3 by T2 describes anellipse E2 and E3 respectively, the intersection of which gives therelative position of the object 34.

It will be obvious that the two ellipses intersect at two points whichcan provide ambiguity in the position of object. Such ambiguity can besolved by adding information from additional tags or by transmittingalso from T2 to T3 and measuring d2+d3.

In order to save power, the tags 4 may be configured with a distributedsynchronization algorithm that puts all their active windows in anoverlapping time frame. In order to limit the occurrence of collisions,the number of concurrent transmitting tags should be limited. Forexample each tag may be configured to transmit in a probability relativeto its number of neighbors.

It is noted that the DSP algorithm described above is only an example.More advanced pattern recognition algorithms can be applied to betteridentify the object.

There is also an option to include additional sensors, at additionalcost, and integrate their data.

It will be appreciated that the above descriptions are intended only toserve as examples and that many other embodiments are possible withinthe spirit and the scope of the present invention.

1. A system capable of monitoring an area and detecting a disturbancewith the area, the system comprising a plurality of ultra-wide bandradio frequency tags, each of said plurality of tags communicating withone or more neighboring tags, and at least one of said plurality of tagsincluding a digital signal processing module configured to monitorchanges in radio frequency multipath properties of received packetstransmitted by at least one other of said one or more neighboring tags,said changes being caused by a disturbance indicative of an object inthe vicinity of the tags.
 2. The system of claim 1, wherein at least onefrom the group including at least one of said tags, at least one readeroperatively associated with said plurality of tags, and a centralprocessor operatively associated with either said reader or said atleast one tag, includes a pattern recognition module which classifiesthe object and determines a location of the object causing saiddisturbance based on said processes changes in radio frequency multipathproperties.
 3. The system of claim 2, wherein at least said centralprocessor is configured to provide information to a user regarding saiddisturbance.
 4. The system of claim 1 wherein a mesh network formedbetween at least some of said plurality of tags.
 5. The system of claim4 wherein said mesh network includes at least one of a reader and acentral processor operatively associated with said mesh network.
 6. Amethod for detecting the entry of an object into a monitored area, themethod comprising: deploying a plurality of ultra-wide band radiofrequency tags, each of said plurality of tags communicating with one ormore neighboring tags and at least one of said plurality of tagsincluding a digital signal processing module configured to monitorchanges in radio frequency multipath properties; monitoring, by at leastone said tag, radio frequency multipath properties of received packetstransmitted by at least one other of said one or more neigh boring tags;and determining if any changes in said frequency multipath propertieshave occurred.
 7. The method of claim 6 further comprising classifyingand determining a location of a disturbance causing said any changes insaid frequency multipath properties using a pattern recognition moduleoperatively associated with any one of at least one of said tags, areader, and a central processor each operatively associated with a meshnetwork.
 8. The method of claim 7, wherein said determining a locationof a disturbance causing said any changes, in said frequency multipathproperties is implemented using said frequency multipath properties fromas few as two said tags.
 9. The method of claim 6, further comprisingforming a mesh network between at least some of said plurality of tags.10. The method of claim 8, further comprising operatively associating atleast one of; a reader and a central processor with said mesh network.11. The system of claim 1 further comprising each of the plurality oftags configured to transmit data regarding one or more multipathcomponents upon detecting one or more changes in radio frequencymultipath properties of received packets.
 12. The method of claim 6further comprising transmitting, by at least one said tag, dataregarding one or more multipath components upon determining one or morechanges in radio frequency multipath properties of received packets. 13.The system of claim 1 further comprising each of the plurality of tagsconfigured to transmit the changes in radio frequency multipathproperties of received packets to a reader or a central processorassociated with the plurality of tags; the reader or the centralprocessor configured to: determine changes in a multipath patternassociated with the plurality of tags based on the changes in radiofrequency multipath properties; and determine characteristics of theobject causing the changes in the multipath pattern.
 14. The method ofclaim 6 further comprising: transmitting, by each of the plurality oftags, the changes in radio frequency multipath properties of receivedpackets to a reader or a central processor associated with the pluralityof tags; determining, by the reader or the central processor, changes ina multipath pattern associated with the plurality of tags based on thechanges in radio frequency multipath properties; and determiningcharacteristics of the object causing the changes in the multipathpattern.